Default theme - What is it and why can't I delete it
From Online Manual
The "Default" theme is actually not the "default theme." While SMF starts out with it as the base theme, the "Default" theme is only a collection of the default templates. These templates are "fallbacks" in case the current theme does not have the correct template.
It is similar to URLs - if the base domain name isn't specified, (in example, just /style.css) it uses the default (making the URL, www.simplemachines.org/style.css). In this case, it works a bit different, but the concept is similar.